This invention relates to thread structures and more particularly to a cylindrical thread structure for use on percussion drilling parts, such as drill strings, which are also rotated.
Such thread structures must provide a firm connection between the elements so that drill rods are not lost down holes and provide a sufficiently tight joint to transfer the drilling impact from rod-striker face to butting rod striker face with best possible efficiency while at the same time should allow the thread to be unscrewed easily. Percussion drilling methods have developed the need for more powerful drilling machines with the resulting higher drilling torque. Many drill units, especially of the percussion variety, utilise threaded connections between the drill rod elements which consist of two or more aligned drill rods with a coupling sleeve. Many drilling methods effect a tightening of the threaded drill elements during use, especially in percussion drilling operations. The torque required for separation of the elements should not be too large for stripping, that is be too difficult to separate the elements after a high torque application which has been applied simultaneously with a percussive load. A further problem with such coupling threads is that they must have an acceptable wearing life.
The uncoupling torque and coupling torque are usually related to each other and among other things depend on the materials used, hardness of the contacting thread surfaces, the angle of the abutting threads and the pitch of the thread. In general the uncoupling torque increases as the pitch and flank angles decrease. U.S. Pat. No. Re. 27,284 proposes a thread structure to minimise uncoupling torque when the threads have symmetrical abutting and secondary flanks which are arcuate and have a maximum flank angle with respect to the drill rod axes adjacent the rod thread crests in the range 45.degree.-20.degree. and preferably 40.degree.-30.degree. which decreases a zero at the bottom of the thread, has two or more starts and, consequently, has a pitch or lead angle of 5.degree.-14.degree., preferably 7.degree.-11.degree., and the crests of the threads are substantially flat. According to the patent these straight crests ensure that the contact between the matching thread surfaces adjacent the crests of the rod thread is established only at portions of the thread having an angle of inclination to the axis which is substantially different from 0.degree. so that wedging does not occur. It is believed that because of the use of two or more starts whereby a greater pitch angle can be employed than was usual at the date of invention of that patent some decrease of the uncoupling torque could be obtained. However, it seems that the varying flank angles counteracts this effect to some extent. Also it is believed that the wear life of the coupling was not improved because of the thread profile which does not maximise the area of the contacting surfaces or minimise the contact pressures. Also the thread profile does not make full use of the hardened surfaces of the coupling structure, which is usually 0.7 to 1.4 mm thick and much wear would take place in the relatively softer, un-hardened material of the coupling. Finally, the thread structure does not allow for the weight of the coupling sleeve to be decreased even though it is known that the wear rate increases as the weight of the coupling sleeve increases.
This invention seeks to provide a coupling structure which will satisfactorily meet the wearing life requirements of a user while at the same time have acceptable unlocking torque characteristics.